Abstract: Red calcareous Middle Cambrian palaeosols from the upper Moodlatana Formation in the eastern Flinders Ranges of South Australia formed in well-drained subhumid floodplains and include a variety of problematic fossils. The fossils are preserved like trace fossil endichnia but do not appear to be traces of burrows or other animal movement. They are here regarded as remains of sessile organisms, comparable with fungi or plants living in place, and are formally named as palaeobotanical form genera under provisions of the International Code of Botanical Nomenclature. Most common are slender (0.5–2 mm) branching filaments flanked by green-grey reduction haloes within the red matrix of palaeosol surface horizons (Prasinema gracile gen. et sp. nov.). Other axial structures (Prasinema nodosum and P. adunatum gen. et spp. nov.) are larger and show distinctive surface irregularities (short protuberances and irregular striations, respectively). The size and form of these filaments are most like rhizines of soil-crust lichens. Other evidence of life on land includes quilted spheroids (Erytholus globosus gen. et sp. nov.) and thallose impressions (Farghera sp. indet.), which may have been slime moulds and lichens, respectively. These distinctive fossils in Cambrian palaeosols represent communities comparable with modern biological soil crusts.
B iological soil crusts in the distant geological past have long been suspected because of dispersed spores (Gray 1981; Strother 2000), abundance of pedogenic clay (Kennedy et al. 2006), unusually deeply weathered composition of Cambrian sandstones (Dott 2003), carbon isotopic composition of palaeosols (Watanabe et al. 2000) and microbially textured bedding planes (Prave 2002). Middle Cambrian microbial filaments (Southgate 1986) and lichen-like fossils (Fleming and Rigby 1972; Müller and Hinz 1992; Retallack 1994) from phosphorites of western Queensland are in sedimentary facies with evidence of exposure within tidal flats and rock platforms (Southgate 1986), but such marine-influenced communities are not directly comparable with biological soil crusts of modern deserts. Unlike grey marine cherts and phosphorites with permineralized fossils or grey shales with carbonaceous compressions, Silurian to Quaternary red oxidized palaeosols seldom preserve cellular detail of carbonaceous fossils, so efficient is recycling in well-drained soils (Retallack 1998). Nevertheless, post-Silurian oxidized palaeosols commonly preserve impressions of leaves, stems and roots, often with drab mottling from biochemical reduction of buried organic matter (Retallack 1997c). Problematic megafossil traces of life on dry land now come from numerous green-red-mottled palaeosols in the Middle Cambrian, Moodlatana Formation of the Flinders Ranges, South Australia (Text-figs 1–3). The Cambrian palaeosols described here predate the evolution of land plants but nevertheless contain three distinct kinds of enigmatic megafossils: (1) drab-haloed filament traces (Prasinema gen. nov.), (2) quilted spheroids (Erytholus gen. nov) and (3) thalloid impressions (Farghera sp. indet.). Although the biological affinities of these fossils remain uncertain, they provide new guides to the appearance of Cambrian life on land.
Formal naming of fossils aids future investigation and wide recognition, as demonstrated by other problematic fossils. For example, Vendobionta were informally noted by Mawson (1938, p. 259) as ‘fossil impressions resembling brachiopod or bivalve form’, but formal description of five species by Sprigg (1947) was needed before their global distribution and importance as Ediacaran fossils could be appreciated (Fedonkin et al. 2007). The various fossils in Cambrian palaeosols formally named here are preserved as bedding disruptions comparable with some kinds of trace fossil (endichnia of Martinsson 1970) but lack backfills, sequential prints or shapes recording movement or behaviour of motile organisms. Each of the different fossil genera described here is preserved in a different way, but all appear to have been remains of sessile organisms such as fungi, lichens, algae or plants in place of growth. Thus, the nomenclatural system appropriate to these fossils is not that of ichnofossils in the International Code of Zoological Nomenclature (Häntzschel 1975; Ride et al. 1999) but rather of palaeobotanical form genera in the International Code of Botanical Nomenclature (McNeill et al. 2006). Form genera such as Thallites (Walton 1923) and Algites (Seward 1894), for example, are used for fossils with the distinctive dichotomizing form of algal, liverwort or lichen thalli, but whose exact systematic affinities are uncertain, because histological and reproductive structures are not preserved.
All fossils reported here were collected from a large exposure of the upper Moodlatana and lower Balcoracana Formation, within a prominent anticline, north of the big bend in Ten Mile Creek, six miles west of the road to Martins Wells, on Wirrealpa Station, South Australia (31°25′N, 138°94′E). These palaeosols are early Middle Cambrian in age, immediately above upper Moodlatana Formation grey shales with the trilobite Onaraspis rubra (Jago et al. 2006), equivalent to the Oryctocephalus indicus zone (Gradstein et al. 2004). At 3602 m in the composite section in Ten Mile Creek, these palaeosols are 508.8 Ma old in the age model of Retallack (2008).
Unlike thin marine dolomites and shales of the Moodlatana and overlying Balcoracana Formations (Moore 1990), the fossiliferous palaeosols represent dry land in terms of both soil drainage and palaeoclimate (Table 1). Pervasive cracking, haematite, loess-like grain-size distribution and low FeO content of red parts of the palaeosols (Table 2) are evidence of well-drained soils of floodplains and supratidal flats. Crack orientation orthogonal to fluvial palaeochannels (Text-fig. 3) is characteristic of gilgai microrelief of Vertisols (Paton 1974). Nodules of gypsum and micritic, low magnesium calcite at shallow depths within the palaeosols are evidence of semiarid to subhumid Middle Cambrian palaeoclimate. Different kinds of palaeosols (pedotypes) have been interpreted to represent different local conditions (Retallack 2008), ranging from intertidal to fluvial (Table 1). The Irkili pedotype for example has much relict bedding, including flaser and linsen bedding, and prominent calcite geodes after gypsum crystals, as in soils of supratidal flats. The Natala and Viparri pedotypes in contrast have extensively disrupted bedding and subsurface caliche nodules (Text-fig. 3) of floodplain soils. The Mindi pedotype is intermediate between these extremes, with small subsurface gypsum crystals and some persistent bedding, and is interpreted as a high supratidal palaeosol (by Retallack 2008). All the fossils described here are from only three (Mindi, Natala and Viparri) of seven pedotypes known in the upper Moodlatana Formation (Table 1). Both Natala and Viparri pedotypes have a calcic horizon deeper than usual for the Moodlatana Formation and represent a time of subhumid, rather than semi-arid climate, immediately before marine transgression of the basal Balcoracana Formation (Retallack 2008).
Table 1. Palaeosol pedotypes in the Middle Cambrian, Moodlatana Formation, Ten Mile Creek, South Australia.
Time for formation
Thin green mottles (A) in red shale
Fluvial microbial mat, with Prasinema gracilis,
Supratidal-alluvial mud flat
Green mottles (A) over red shale with chalcedony geodes (By)
Arid (100–300 MAP), high evapotranspiration
Intertidal microbial mat, with Prasinema gracilis,
Supratidal and alluvial mud flat
Grey-red-mottled siltstone (A)
Polsterland, with Prasinema gracilis, Erytholus rotundus and Farghera sp. indet.
Supratidal mud flat
Red siltstone (A) over deep (>50 cm) calcareous nodules (Bk)
Subhumid (500–800 mm MAP)
Polsterland, with Prasinema gracilis, P. nodulosa, P. fascicularis
Low alluvial terrace and floodplain
Red siltstone (A) with pseudo-anticlinal sandy layers over deep (>50 cm) calcareous nodules (Bk)
Subhumid (500–800 mm MAP), with marked dry season
Polsterland, with Farghera sp. indet.
Low alluvial terrace and floodplain
Ferruginized sandstone (A)
Polsterland, with Prasinema gracilis,
Alluvial levee and point bar
Red clayey siltstone (A) over shallow (<50 cm) calcareous nodules (Bk)
Polsterland, with Prasinema gracilis,
Supratidal sand flat
Table 2. XRF chemical analyses of red and green samples (weight per cent).
Error is from 10 analyses in same laboratory (ALS-Chemex, Vancouver, BC, Canada) and standard (BC Canada granodioritic stream gravel SDMS-2).
The fossils described here are surprisingly large and plentiful for what would be expected in Cambrian palaeosols (Retallack 2008), and the question may be raised whether they represent biological activity after the Cambrian. They do not appear to be products of modern weathering, Cenozoic lateritization or Permian glacial landscapes because found in deep boreholes: Prasinema is common at 1493–1504 feet, Farghera at 1498–1500 feet in Lake Frome no. 2 core and Prasinema at 2089–2090 feet in Lake Frome no. 3 cores archived in the Primary Industries and Resources South Australia (PIRSA) core library at Glenside, a suburb of Adelaide. In outcrop, these fossils can be found more than a metre back from the surface within rock that has organic matter reflectance and clay-mineral illitization of lower greenschist facies of regional metamorphism (Retallack 2008). All the fossils are in strata-concordant, dipping layers, traceable laterally for about 50 m and at an angle to modern soils and landscapes. The red palaeosols with fossils are also interbedded with unweathered black shales and stromatolitic limestones (Text-fig. 2). These fossils are an integral part of Cambrian soil structures and horizons (Retallack 2008).
Materials and Methods
Fieldwork in South Australia in 2003, 2006 and 2007 included measurement of stratigraphic sections, azimuths from trough cross-beds and palaeosol crack orientations using a Brunton compass, spacing of fossils using a milliners tape and dimensions of fossils using a digital callipers. Samples were thin-sectioned for petrographic observations and analysed for major element composition and iron valence state using XRF and potassium dichromate titration (respectively), by ALS Chemex of Vancouver (Canada), against Canada granodioritic stream gravel standard SDMS-2. Fossil specimens are housed in collections of the South Australian Museum, Adelaide.
Derivation of name. Elided from Greek prasinos (green) and nema (neuter, thread).
Diagnosis. Network of fine (<2 mm diameter) filamentous green-grey, sediment-filled, irregular tubes, radiating and decreasing in abundance downward from a sedimentary surface, clear grey-green reduction haloes around the filaments contrast with red sedimentary matrix; unbranched or branching at irregular intervals and angles, without distinct orders of branch thickness.
Taphonomy. Networks of drab-haloed filaments are common at the surface of both Mindi and Natala pedotype palaeosols (Text-figs 2A, 3), which were probably Aquepts and Calcids, respectively (Retallack 2008) in the US soil taxonomy (Soil Survey Staff 2000). These palaeosols also include green-grey horizons and planar features coating soil structure (Text-fig. 5B), but Prasinema is only applied to tubular, ellipsoidal or elongate structures of irregular form (Text-figs 4D, 6A). Drab-haloed filament traces are especially clear in horizons between the entirely drab surface and red subsurface horizons. These structures grew through the soil, dilating and disrupting primary bedding under low (not deep burial) confining pressures (Text-fig. 6C). A central filament <2 mm in diameter is filled with yellow-green claystone with sharp contacts to green-grey matrix extending outwards to a diffuse contact with red matrix (Text-figs 4C, 5B, 6A–D). Both green-grey halo and red matrix have the same silty petrographic texture, but the filament fill is slightly more silty (Text-fig. 6A–D). Furthermore, the green-grey claystone is not much different in total iron content than the red claystone, though richer in ferrous iron (Table 2), unlike comparable redoximorphic features in Mesoproterozoic (Driese et al. 1995) and Cenozoic gleyed palaeosols (Retallack 1983; Retallack et al. 2000). The green-grey matrix is thus a chemically reduced alteration halo, produced largely during closed-system diagenesis, rather than during open-system gleization in a waterlogged soil, or during introduction of contrasting material before burial (Retallack 2001a). Alteration during burial also is supported by correlation of central filament diameter with halo diameter, generally similar to that known from drab-haloed root traces of trees in Devonian and younger palaeosols (Table 3; Retallack 1997a). Filament traces are more like fine root traces than large woody large root traces in scaling closer to surface area (2πr in two dimensions of cross sections measured) than to volume (πr2 also in cross section: Text-fig. 7).
Table 3. Additional occurrences of Radicites erraticus (drab-haloed root traces).
Cambrian drab haloes are very similar to Devonian and younger root traces, referable to the form species Radicites erraticus (Arafiev and Naugolnykh 1998), and thought to have formed through gleization of iron oxides and hydroxides by dysaerobic microbes consuming remnant soil organic matter shortly after burial (Retallack et al. 2000). The drab-haloed filaments have top-down gleization like that of surface-water gley, rather than bottom-up gleization of groundwater gley (Retallack 2001a). Surface-water gley is unlikely because grain-size distributions of Cambrian palaeosols failed to detect any fine-grained or cemented impermeable horizon (Retallack 2008) that would perch water table. The most likely explanation is formation of gleyed haloes during microbial consumption of organic matter immediately after burial of the palaeosol and relative rise of water table in a subsiding floodplain. By this view, drab-haloed filaments represent the last crop of the palaeosol before burial, but red filamentous structures of comparable size (Text-fig. 4D) represent organisms that died and decayed within the oxidizing environment of the original soil.
Also generally similar to Prasinema are drab filaments from palaeosols of the 1.8 Ga Lochness Formation, near Mt Isa, Queensland (Driese et al. 1995), which differ in having much less total iron in the drab than red palaeosol matrix, and haloes of local iron enrichment. Comparable Cenozoic drab-haloed pedotubles with iron–manganese bands have been interpreted as root traces of plants adapted to seasonally waterlogged palaeosols, which lost ferrous iron to groundwater (Retallack 1983). Unnamed tubular Precambrian fossils with green-grey haloes in red quartzites from the 2.0 Ga Medicine Peak Quartzite of Wyoming are described as stouter and more bluntly ending than Prasinema (by Kauffman and Steidtmann 1981).
Biological affinities. Prasinema was a large organism for Cambrian palaeosols, with filaments extending as much as 30 cm down into palaeosols and interconnected into larger networks. Thin sections (Text-fig. 6) betray no evidence for biomineralization in Prasinema. The taphonomic model for these and other drab-haloed tubular structures (Retallack et al. 2000) proposes that the filaments were made of organic carbon compounds, later consumed as fuel for biological reducing power to create the drab haloes. Furthermore, the log-normal (negatively skewed) size distribution of both Prasinema and Radicites diameters is evidence of indeterminate growth, as has been argued for other fossils comparable with colonial animals, plants and fungi (Peterson et al. 2003).
Biological soil crusts contain a variety of unskeletonized organisms comparable in size, structure and growth with Prasinema: adventitious roots of grasses and other herbaceous tracheophytes, rhizoids of liverworts or mosses, bundles of filamentous cyanobacteria (Microcoleus), fungal hyphal bundles or lichen rhizines (Belnap and Lange 2003). Roots or rhizoids are unlikely because longer and shorter, respectively, and also forming a sharper boundary with soil matrix than apparent in thin section (Text-fig. 6A–D). Incorporation of soil matrix within the central tubular hole within the drab halo is more like bundles of cyanobacteria and fungal hyphae. A hyphal origin is most likely considering the deep reach (30 cm) of these structures within the palaeosols, well beyond the surficial zone of light penetration, because taphonomic evidence for burial gleization discussed above is an indication that drab-haloed Prasinema is the current crop of organic structures, as in comparable drab-haloed root traces (Retallack et al. 2001). Very similar structures to Prasinema are hyphal bundles of non-lichenized fungi, such as the bootstrap fungus (Armillaria mellea: Mihail and Bruhn 2005) and rhizines of Ascolichens and Basidolichens, especially placoid forms (Vogel 1955; Poelt and Baumgärtner 1964). These two alternatives cannot be distinguished on morphological grounds, although primary productivity of Cambrian palaeosols lacking embryophytic plants would be unlikely to support such abundant hyphae of non-lichenized fungi.
Holotype. Near-vertical specimen, concertina-shaped from burial compaction in the centre of the saw slab (P42257; left-hand side; Text-fig. 4D; right-hand side; Text-fig. 5B), from the type Mindi palaeosol, upper Moodlatana Formation, at the big bend in Ten Mile Creek, South Australia (31°25′N, 138°94′E).
Other localities. This is the most abundant fossil in Cambrian palaeosols of South Australia and is found in almost all outcrops and drillcore of the Parachilna, Billy Creek, Moodlatana and Balcoracana Formations, and in some outcrops of the Pantapinna and Grindstone Range Sandstones. Prasinema gracile thus ranges in geological age from earliest Cambrian perhaps to Early Ordovician in the Flinders Ranges of South Australia (Retallack 2008).
Derivation of name. Latin gracilis meaning slender.
Diagnosis. Prasinema with fine (<1 mm) filaments, flanked by a drab halo extending a comparable thickness outward into reddish matrix; slender, striated and flexuously bent; branching irregularly and sparsely, without orders of branching; permeating rock matrix and destroying primary sedimentary structures with no clear preferred orientation.
Dimensions. Mean diameters (±standard deviation, range) of 237 filaments from the Mindi palaeosol were 0.56 (±0.29, 0.03–1.58) mm, and their haloes were 1.73 (±0.78, 0.43–5.80) mm (Text-fig. 7). Comparable diameters of 430 filaments in the Natala palaeosol were 0.50 (±0.25, 0.12–2.13) mm, and their haloes were 1.46 (±0.69, 0.47–7.12) mm.
Comparisons. These conspicuously drab-haloed tubular features are relatively nondescript compared with other species of Prasinema with stouter (>2 mm) and less flexuous filaments (P. adunatum) and lateral thickenings (P. nodusum). Sparse and irregular branching of Prasinema gracile, ramifying in all directions through the rock, obscures primary sedimentary structures, which are prominent in beds abruptly overlying the palaeosol, and also lower in the palaeosol where Prasinema is sparse (Text-fig. 5A).
Holotype. Left-hand example illustrated (Text-figs 4F, 8B) in specimen P42310 from Natala palaeosol in Ten Mile Creek.
Derivation of name. Latin nodosus meaning knotty.
Diagnosis. Prasinema with fine (<1 mm) filaments densely invested with outwardly directed emergences, varying from globose to spinose in shape, and narrow green-grey halo into red matrix; unbranched and straight; subvertical preferred orientation.
Description. These fossils are rare, and their relationship with the slender filaments is unclear. The spacing of emergences ranges from tightly clustered (Text-fig. 4F) to well spaced (Text-fig. 6D), so that it is conceivable that these are fertile or specialized segments of Prasinema gracile. Unlike P. gracile, which runs in all directions in the rock, P. nodosum was only found near vertical to bedding planes defining the tops and bottoms of enclosing palaeosols. In thin section, they include drab-coloured matrix comparable in texture with the red matrix, and so these are interpreted as organisms in place of growth within the soil, rather than parts of organisms protruding from and then onlapped by upbuilding soil.
Dimensions. Mean (±standard deviation, range, number of measurements) include central filament diameter of 0.95 mm (±0.05, 0.91–0.99, 2), external diameter of 2.51 mm (±0.56, 2.11–2.90, 2) mm and external thickening diameter of 0.72 mm (±0.08, 0.61–0.91, 11) mm.
Comparisons. Middle Cambrian unnamed phosphatized tubes with sharp lateral extensions from the Beetle Creek Formation at Mt Murray, Queensland (Fleming and Rigby 1972), differ in mode of preservation, are hollow with crushing indicative of horizontal preservation and about twice the size of P. nodosum. Nevertheless, both fossils have a striated or filamentous construction and lateral spines, which may reflect comparable biological affinities.
Less similar to Prasinema nodosum are other congeneric species, which either lack the outer thickenings (P. gracile) or are much thicker (P. adunatum). Prasinema nodosum has a superficial resemblance to a spinose plant, such as the fossil moss Muscites guelescini (Anderson and Anderson 1985), the zosterophyll Sawdonia ornata (Gensel 1991), or the putative alga Margaretia dorus (Gunther and Gunther 1981), but unlike these does not appear to have a finished cellular epidermis or cuticle. This same objection also distinguishes Prasinema from problematica that have been regarded as mosses or lycopsids, such as the Cambrian ‘Aldanophyton’ (probably junior synonym of Margaretia according to Rozanov and Zhuravlev 1992), and Ordovician Akdalophyton (Snigirevskaya et al. 1992) and Boiophyton (Obhrel 1959). Like Prasinema, Mesoproterozoic Horodyskia moniliformis is also found as grey-green markings within purple-red siltstones (Fedonkin and Yochelson 2002; Martin 2004; Fedonkin et al. 2007, p. 33) but has the appearance of beads loosely strung on a thread, rather than the clustered thickenings of Prasinema nodosum.
Holotype. Single thick axis on specimen P42313 (Text-fig. 8C); from the type Natala palaeosol in Ten Mile Creek.
Derivation of name. Latin adunatus meaning united.
Diagnosis. Prasinema of stout (2 mm) filaments, with a striated appearance and irregular swellings and thinnings; unbranched and subhorizontal in orientation.
Description. This axis runs oblique to relict bedding planes but is closer to horizontal than vertical within the bounding surfaces of the type Natala palaeosol. Branching of the axis was not seen, nor is there any clear connection with closely associated filaments of Prasinema gracile. Like P. gracile, P. adunatum has a similar sedimentary texture in both drab-coloured material and surrounding red matrix.
Dimensions. The single specimen found ranged from 2.10 to 2.45 mm wide (mean 2.32 mm, standard deviation 0.18 mm).
Comparisons. Middle Cambrian phosphatized tubes (unnamed) from the Beetle Creek Formation at Mt Murray, Queensland (Fleming and Rigby 1972), are similar to P. adunatum in size, longitudinal striation and sub-horizontal preservation but differ in their prominent lateral spines. Prasinema adunatum differs from both P. gracile and P. nodusm in larger diameter, which appears to be the result of numerous subparallel filaments of comparable form. Cambrian marine algae such as Yuknessia simplex (Gunther and Gunther 1981) differ in showing dichotomous branching, cellular margins and preservation as organic compression within bedding planes.
Derivation of name. Elided from Greek erythros (red) and tholos (masculine, dome).
Diagnosis. Spheroidal sandy and silty structures, with median vertical column and glide symmetry of 4–12 subhorizontal internal partitions; a thin axial thread within the central column extends vertically above and below the spheroid.
Taphonomy. These distinctive quilted spheroids within the type Mindi palaeosol are comparable with trace fossil endichnia (of Martinsson 1970), yet there is no evidence of animal movement. They are entirely oxidized. Although surrounded and penetrated by drab-haloed filaments of Prasinema gracile, there is no thickening, curvature or other accommodation of drab filaments suggestive of relationship between P. gracile and Erytholus globosus. An axial thread seen in many specimens of E. globosus is always red (Text-fig. 4G–J), never grey-green like Prasinema. Erytholus can be observed in outcrop on vertical faces, and by cracking open rock. Most specimens shatter through the middle to reveal subhorizontal chambers (Text-figs 4G, I, 5F); very few expose the strongly curved, thick walled, outer surface (Text-figs 4J, 5E). Internal quilting was not a bedded cavity fill or internal mould (colloform illuviation argillan, of Retallack et al. 2000, fig. 88), burrow backfill (Retallack 2001b) or internal chambering or backfill of a sediment-ingesting organism (Seilacher 1992; Savazzi 2007), because it cannot have been moved within the whole structure, as revealed by the following observations. In one vertical face, two Erytholus were exposed by breaking open the rock vertically and recording former orientation (Text-fig. 5F). The upper smaller Erytholus has a more sandy upper than lower zone, and the lower large one has a more clayey central zone, with sandy upper and lower portions. These lithological differences are seen also as beds in the immediately flanking surrounding matrix. Thus, the internal organic quilt grew within the sediment or was a hollow structure filled with sediment increments, without moving sediment far from original layering, as has been suggested for some Ediacaran fossils (Grazhdankin and Seilacher 2002; Grazhdankin 2005). The taphonomic mode of Erytholus can be interpreted in two distinct ways: (1) internal mould or (2) sand skeleton. By the sand skeleton interpretation, the organism grew within the sediment, but by the internal mould interpretation, the chambered organism lived (and died?) at the surface and was later infiltrated by increments of local sediment.
Comparison. When first discovered, Erytholus specimens were suspected to be enrolled trilobites or aglaspids. However, thin sections and sawn slabs (Text-figs 5D, 6F) revealed no exoskeletal remains, doublure, axial fold or limb impressions. The axial column runs vertically through the middle of the structure and is not curved around the periphery, as in an enrolled trilobite. Other similar ovoid structures in red beds are Ediacaran fossils with comparable gliding plane symmetry and quilting (Tojo et al. 2007), such as Ernietta, Pambikalbae and Ventogyrus. Ernietta is hollow, rather than a three-dimensional internally layered structure, with internal column and threads, and this hollow is filled with white quartz sandstone distinct from the red siltstone matrix (Dzik 1999). Pambikalbae has a central column and lateral chambers but is much larger (>29 cm long) and more elongate than Erytholus (Jenkins and Nedin 2007). Like Erytholus, Pambikalbae is preserved in three dimensions within red sandstones. Ventogyrus is ovoid, with a central column and thread, lateral threads and thick wall, but differs from Erytholus in its distinct trigonal vertical divisions and is also larger (up to 6 cm diameter and 12 cm long; Fedonkin and Ivantsov 2007). However, Ventogyrus is preserved with central thread and column vertical to bedding in red-mottled fine sandstones (Fedonkin et al. 2007, pp. 142–145; D. Grazhdankin, pers. comm. 2008), comparable with Erytholus in the Mindi palaeosol. These similarities of preservation and internal structure suggest that Erytholus may be grouped with Pambikalbae and Ventogyrus within the problematic group Vendobionta (Seilacher 1992, 2007).
Biological affinities. Fedonkin and Ivantsov (2007) regarded the comparable vendobiont Ventogyrus as a siphonophore cnidarian (comparable with the ‘bluebottle’, Physalia physalis), and Dzik (2003) compares Ventogyrus with ctenophore cnidarians (‘comb jellies’, such as Cestum veneris). Ctenophores are known as flattened impressions back at least as old as Early Cambrian, in the Chengjiang fauna of China (Hou et al. 2004), so their fossil record does not rest entirely on interpretation of controversial vendobionts. Neither of these groups of hollow, flimsy, marine organisms is a suitable explanation for fully inflated and little-deformed specimens of Erytholus in a palaeosol, filled with sediment preserving exterior bedding.
In contrast, Seilacher (2007; Seilacher et al. 2005) regards vendobionts such as Dickinsonia and Palaeopascichnus as xenophyophores, comparable with the giant (up to 25 cm) Stannophyllum zonarium of deep marine sediments. An appealing aspect of this interpretation is the included sediment (xenophyae), faeces (stercomare) and exudates (barite) within xenophyophores (Levin 1994), comparable with observations of Erytholus in thin section (Text-fig. 6F). However, xenophyophores have irregular or meandrine chambers and lack internal organization of central thread within a vertical column, and flanking chambers seen in Erytholus. No xenophyophores are known in soil or nonmarine settings. The fossil record of xenophyophores, other than controversial Vendobionta, is equally controversial: trace fossils (Palaeodictyon and other graphoglyphid traces) no older than Early Cambrian, and calcareous skeletonized forms (so called ‘phylloid algae’) no older than Carboniferous (Levin 1994).
Other plausible biological models for the whorled filamentous construction of Erytholus are green algae, particularly Charales known back to Early Devonian (Feist and Feist 1997), or Dasycladales such as Chaetocladus known back to Middle Ordovician (Kenrick and Vinther 2006). Such algae are aquatic and not known from palaeosols but could conceivably have been a part of the aquatic parent material of the Mindi palaeosol, as in an enigmatic calcite-filled axis from Ordovician red beds of the Juniata Formation in Pennsylvania (Retallack 2001b). This enigmatic fossil from Pennsylvania, like Charales and Dasycadales, was a system of dichotomously branching tubes arranged in whorls. In contrast, Erytholus is not constructed as a whorled scaffolding but quilted from planar to filamentous partitions without true whorling, and a glide symmetry of offset laterals.
Another possible biological model for Erytholus is a truffle, meant here in the general sense of underground fungus, rather than implying the commercial extant species Tuber melanosporum (Pezizales, Ascomycota). Truffle growth form evolved independently in several fungal lineages: Zygomycotina (pea truffles), Ascomycota (true truffles), Basidiomycota (false truffles) and Deuteromycota (anamorphous fungi: Bruns et al. 1989; Pegler et al. 1993). Truffles have internal chambers in a variety of patterns: radial, alveolar and spongy. Radial–bilateral chambers most like Erytholus are known from Elaphomyces muricatus (Elaphomycetales, Ascomycota: Pegler et al. 1993), but this lacks a central column or thread. There is a possible Precambrian fossil record of Ascomycota (Retallack 1994, 2007), ‘higher fungi’ (Ascomycota + Basidiomycota: Butterfield 2005) and Glomeromycota (Yuan et al. 2005), so that these Cambrian fossils would not be unusually old fungi. Nevertheless, all truffles exclude sediment from their interior, and although it could infiltrate chambers of decayed examples, the abundant included sediment continuous with exterior grain-size variation makes truffles an unlikely explanation for Erytholus.
A final possibility for Erytholus is a sporangium of a slime mould (Myxomycota), traditionally regarded as related to fungi (as ‘myxomycetes’), but now regarded as more closely allied to Amoebozoa (of Baldauf 2003). These creatures are generally dispersed in the soil as flagellated or amoeboid cells or as an irregularly shaped multinucleate plasmodium (Stephenson and Stempen 1994), but the stalked sporangia have a variety of internal structures similar to Erytholus. A reproductive rather than vegetative organ is suggested by the near-normal distribution of sizes (Text-fig. 9), unlike the skewed distribution of Prasinema (Text-fig. 7) and other fossils of indeterminate growth (Peterson et al. 2003). In the slime mould Physarum crateriforme, for example, the stalk of the sporangium continues up within the spheroidal mass of sporangia as a columella, which gives off a network of lateral filaments (capillitium) defining crude chambers within an outer thick wall (peridium: Martin et al. 1983). Such structures release spores into the air above the ground and, for Erytholus, would imply growth from the soil surface, with later covering and infiltration by increments of aeolian or waterlain silt. Such a gap in time for decay of organic matter is compatible with the lack of drab haloes around Erytholus, in contrast to the taphonomy of what are here interpreted as freshly buried Prasinema. Putative slime mould compression microfossils have been reported from the 1.025 Ga Lakhanda Group of Siberia (Hermann and Podkovryov 2006), and problematic trails of about the same age from the Chorhat Sandstone of India may have been created by slime moulds (Conway Morris 2000), so neither Erytholus nor comparable Ediacaran Ernietta, Ventogyrus, or Pambikalbae would be the oldest fossil record of such organisms. Differences between Erytholus and slime mould sporangia include an order of magnitude larger size and continuation of the axial thread out the top of the structure.
Holotype. Large lower example of specimen P42255 (Text-fig. 5D, F); from the type Mindi palaeosol in Ten Mile Creek (top of specimen was marked by a black circle coplanar with the ancient land surface).
Derivation of name. Latin globosus meaning spherical.
Diagnosis. Erytholus 5–20 mm in diameter, with 4–12 stacked internal layers divided by a wide (4–6 mm) vertical column.
Description. Erytholus spheroids are smooth or sparsely ridged when cracked out of the rock in exterior view, but, in cross section, show an irregular system of subhorizontal chambers filled with red claystone and white sandstone. The chambers have the general appearance of bilateral symmetry around a central column but, in detail, are not entirely symmetrical, with horizontal quilting at slightly different levels and chamber margins turning either up or down at the margins (Text-fig. 4). The chambers are also ill-defined by ferruginized claystone (Text-fig. 6F). The chamber floors are deflected where they meet the central column, but a narrow tubular structure within that extends both above and below the structure for an undetermined distance.
The size distribution of Erytholus, and the number and size of its chambers are near normal (Text-fig. 9A–D). Burial compaction has rendered them slightly oblate, on average, so that width is generally greater than height (Text-fig. 9E). Chamber thickness does not vary with overall width except in the smallest specimens, but this relationship does not have the statistical significance expected of growth relationships of metazoans (Text-fig. 9F).
The distribution of Erytholus within the type Mindi palaeosol is highly variable, from barely a centimetre apart (Text-fig. 5F) to more than a metre. Average spacing of 78 specimens seen in outcrop along 62 m strike length of the Mindi palaeosol was 1.19/m. All were in the upper 20 cm of the palaeosol, which has relict bedding indicative of a cumulic A horizon (Retallack 2008), and thus supportive of a taphonomic model of a surface hollow structure filled by increments of sediment.
Dimensions. Mean (±standard deviation, range) of 145 specimens of Erytholus globosus include horizontal diameter (coplanar with bedding) of 13.09 mm (±4.82, 3.36–34.77), vertical diameter 10.25 mm (±3.65, 2.85–24.18), chamber height 1.53 mm (±0.43, 0.46–2.57) and number of chambers 7.45 (±1.10, 5–12).
Localities. Most of the fossils of Erytholus globosus were found in the type Mindi palaeosol in the uppermost Moodlatana Formation at 3602 m in the composite section in both Ten Mile (31°25′S, 138°94′E) and Balcoracana Creeks (31°18′S, 138°90′E), but others were seen at the Ten Mile Creek locality in the Irkili palaeosol of the lower Balcoracana Formation at 3611 m.
Comparisons. Only one species of Erytholus is currently recognized.
Description. Viparri palaeosols have disrupted surficial sandy layers and deep cracks oriented orthogonal to palaeochannel direction (Text-fig. 3) like those of modern swelling clay soils, or Vertisols (Soil Survey Staff 2000). These light-coloured sandstone stringers give good contrast between thin structures filled with red clay and with regular dichotomous branches radiating from a centre, spreading upward at low angles to relict bedding (Text-figs 4A–C, 6E, 8G–I). These specimens were examined under an environmental scanning electron microscope (FEI QANTA capable of forming an image without coating), and only clay was seen, with no histological details. Thin section examination confirms that these are impressions filled with clayey sediment and have shelf-like or tubular extensions (Text-fig. 6E).
Dimensions. Mean width (±standard deviation, range) of 500 specimens of Farghera sp. indet. is 1.78 mm (±0.58, 0.53–3.68, see Text-fig. 10).
Taphonomy. The preservational style of these dichotomizing fossils is identical with plant impressions preserved in red palaeosols in growth position, such as leaves of Evolsonia from the Permian of Texas (Mamay 1989) and Sanmiguelia from the Triassic of Colorado (Tidwell et al. 1977). Lack of histological details rules out the taphonomic model of Spicer (1977) in which a replica of the leaf surface is made by predepositional ferric oxide coatings fuelled by microbial decay. This latter model applies best to Cretaceous Araliaephyllum leaves from swales of seasonally waterlogged palaeosols in sandy levees in the Dakota Formation of Kansas (Retallack and Dilcher 1981). Viparri palaeosols in contrast show cracking patterns and orientations suggestive of well-drained soils (Text-fig. 3).
Comparisons. Impressions and compressions of dichotomously branching thalli are commonly assigned to the form genera Thallites (Walton, 1923) and Algites (Seward, 1894), but Farghera differs from both form genera in its rhizine-like structures scattered along the margins of the thallus, and occasional monopodial and irregular branching (Retallack 2009). The only species of Farghera known so far is F. robusta, which has rounded thallus terminations about half the width of these specimens from the Viparri palaeosol. Viparri specimens are more fragmented and also represent a larger thallus of more wrinkled form. The broad thallus may be an indication of a more mesophytic form than F. robusta known from sandy Entisol palaeosols (Upi pedotype associated with Adla and Matarra Aridosols) of drier climate than Vertisols (Viparri of Retallack 2008). This material is a different species than Farghera robusta, but detailed characterization will have to wait discovery of more complete and informative material.
Biological affinities. Comparable dichotomizing thalli are found in liverworts such as Marchantia (Smith 1990) and algae such as Fucus and Dictyota (Graham and Wilcox 2000), but these lack the rhizine-like extensions characteristic of Farghera (Retallack 2009). The Viparri thalli are comparable with foliose lichens such as Xanthoparmelia reptans and Physcia caesia (Text-fig. 11B). Small lichens of ground-hugging rosette growth habit are common in biological soil crusts of modern deserts (Belknap and Lange 2003). Farghera would not be the oldest known lichen, because putative permineralized lichens are known from the 0.6 Ga Doushantuo Formation of China (Paramecia among others, as interpreted by Retallack 1994; unnamed crustose form of Yuan et al. 2005) and also the 2.6 Ga Carbon Leader of the Witwatersrand Group of South Africa (Thucomyces of Hallbauer and Van Warmelo 1974; Hallbauer et al. 1977). Other fossil lichens include Devonian crustose (Taylor et al. 2004) and foliose forms (Jurina and Krassilov 2002), Eocene microscopic epiphyllous forms (Sherwood-Pike 1985) and Oligocene fragments in amber (Poinar 1992).
Biological soil crust is a term introduced by Belnap and Lange (2003) because such desert vegetation includes microbes (cyanobacteria and algae), microbial fruiting bodies (mushrooms and slime moulds), vascular cryptogams (lycopsids and ferns) and nonvascular plants (mosses and liverworts). Such wide definition of biological soil crusts thus includes (1) microbial earths, recognized by stromatolitic and other microbial textures, (2) polsterlands, recognized by discrete megascopic nonvascular forms, and (3) brakelands, recognized by megascopic herbaceous vascular plants other than grasses (Retallack 1992). Cambrian polsterlands are thus indicated by this paper, which reports three problematic kinds of megascopic remains comparable with those of lichen thalli and rhizines, and slime mould fruiting bodies from Cambrian palaeosols (Text-fig. 11). Weathering, carbon sequestration and landscape stabilization under modern polsterlands are modest compared with those under vascular land plants, but far from negligible (Retallack 1992), as indicated also by petrographic and geochemical study of Cambrian palaeosols (Retallack 2008).
Lack of water, heat and essential nutrients is an important limit to productivity of modern biological soil crusts in deserts, but they thrive also in warm-wet regions until outcompeted by other plant communities (Belnap and Lange 2003), such as brakelands dating only back to Early Silurian and woodlands dating back to Middle Devonian (Retallack 1992). An important limit to life on land on the early Earth was ultraviolet light, especially before about 2 Ga when oxygen levels were less than 0.1 times modern level, too low to create a significant ozone layer (Kasting 1987). Drab-haloed filament traces in red oxidized soils comparable with those reported here have been described (though not interpreted as biological soil crusts) from the 1.8 Ga Lochness Formation of western Queensland (Driese et al. 1995). Even with significant ultraviolet radiation, life could survive within soil at levels where hard radiation was filtered by overlying transparent grains (Sagan and Pollack 1974), so that the antiquity of drab-haloed root traces or other evidence of life in palaeosols may not be a reliable guide to past variation in Earth-surface ultraviolet radiation.
In summary, a long suspected fossil record of polsterlands in pre-Devonian rocks now includes a variety of megafossils in surface horizons of moderately developed Cambrian palaeosols representing stable land surfaces of floodplains and supratidal flats (Text-fig. 12). These megafossils include drab haloes around filamentous structures, chambered spheroids and thalloid impressions. Comparable structures may be widely overlooked in pre-Devonian red beds, and this account provides search images, a taxonomic framework and an introduction to their interpretation.
Acknowledgements. Pauline Coulthard offered advice on aboriginal sacred sites, and Barbara and Warren Fargher graciously gave permissions for fieldwork on Wirrealpa Station. Research was funded by the Petroleum Research Fund of the American Chemical Society, and fieldwork aided by Christine Metzger.